Flame detecting apparatus

ABSTRACT

In apparatus wherein an AC power source and a light emitting device are connected in series through a lead between a burner and a flame rod disposed in a place in which a flame is to exist, so as to detect the presence or absence of the flame in the form of the presence or absence of the output of light emission of the light emitting device, a flame detecting apparatus comprising a capacitor connected in parallel with the light emitting device, thereby to prevent a malfunction attributable to the stray capacity between the lead and ground, the deterioration of the insulation between the flame rod and the burner, or the like.

United States Patent Ikegami et a].

[ FLAME DETECTING APPARATUS [75] Inventors: Kazutoshi Ikegami; Mitsuoki Yamamoto, both of Tokyo; Kenji Toudo; Motosi Miyanaka, both of Yanai, all of Japan [73] Assignee: Hitachi, Ltd., Tokyo, Japan [22 Filed: Jan. 8, 1973 [21] Appl. No.: 321,967

[30] Foreign Application Priority Data Apr. 28, 1972 Japan 47-42224 Apr. 14, 1972 Japan 47-43447 Apr, 5,1972 Japan...., 47-33519 [52] US. Cl. 431/79, 317/130 [51] Int. Cl. F23n 5/08 [58] Field of Search 431/78, 79; 317/130 [56] References Cited UNITED STATES PATENTS 3,348,104 10/1967 Zielinski et al. 431/79 Y TIMER FL C [ Sept. 17, 1974 3,574,496 4/1971 Hewitt 431/79 Primary Examiner-Edward G. Favors Attorney, Agent, or Firm-Craig & Antonelli [5 7] ABSTRACT In apparatus wherein an AC power source and a light emitting device are connected in series through a lead between a burner and a flame rod disposed in a place in which a flame is to exist, so as to detect the presence or absence of the flame in the form of the presence or absence of the output of light emission of the light emitting device, a flame detecting apparatus comprising a capacitor connected in parallel with the light emitting device, thereby to prevent a malfunction attributable to the stray capacity between the lead and ground, the deterioration of the insulation between the flame rod and the burner, or the like.

16 Claims, 8 Drawing Figures PAIENIEB I 71974 .SHEEI 1 0F 2 FIG. 2

loboo B who CAPACITANCE (pF) IOO 1 FLAME DETECTING APPARATUS BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to apparatus for detecting flames, and more particularly to flame detecting apparatus in which a light emitting device such as a discharge tube and an electroluminescent diode is turned on and off in response to the presence and absence of a flame, thereby to convert, in essence, the wavelength of the light of the flame into the substantially constant wavelength of the light of the light emitting device, whereupon the light of the constant wavelength is converted into an electric signal by means of a photocell or the like.

2. Description of the Prior Art Apparatus for detecting the presence or absence of the flame of a burner has heretofore been suggested and put into practical use in various systems. One of the systems consists in that the light of the flame is directly converted into an electric signal by the use of a photocell, phototransistor or the like. The system is disadvantageous in that, since the wavelength of the flame differs in dependence on the kind of fuel such as oil and gas, a separate detector need be used for each fuel. In case of the combustion of gas, the wavelength of the flame is much shorter than in the combustion of oil, so that the detection is extremely difficult with the photocell or the like. In the system, usually, the photocell is mounted on a monitor window of a furnace, and the presence or absence of the flame is converted into an electric signal outside the furnace. The system accordingly has the problem that, when it is used for many years, the detection sensitivity of the photocell lowers due to soot, dust etc. adhering to the monitor window.

Another prior-art system consists in that the presence or absence of the flame is directly converted into an electric signal by making use of the electric conductivity or rectifying effect of the flame. A detector in this case is constructed such that anAC or DC voltage is applied across the burner and an electrode disposed in a place where the flame is to exist, and that current flowing through the closed circuit is detected. Herein, since the internal impedance of the flame is extremely high, the input impedance of an amplifier for amplifying the detected current is required to be so high as is comparable with the internal impedance. As an amplifier device satisfying the requirement, the insulated gate field-effect transistor is known. However, when the PET is connected with the electrode inserted into the flame of the high internal impedance, noises mixed.

into the input of the F ET through the stray capacity of the lead of the connection raise a problem. A further disadvantage is that, since the electrode inserted into the flame at a high temperature and the FET are directly connected by the lead, the heat of the flame is conducted to the FET to raise the temperature thereof and to accordingly change the characteristics of the FET largely.

In case of utilizing the rectifying effect of the flame, that is, the property that, in the electric conductivity of the flame in the vertical direction, one part is smaller in the electric resistance than another part, since the direction of the rectification characteristic of the flame varies due to burning conditions etc., the design of 21 bias circuit for the FET is difficult, and malfunctions are often caused.

Apparatus has also been suggested in which a light emitting device is operated in response to the presence or absence of the flame, and the emanating light output of the device is converted into an electric signal, thereby to detect the presence or absence of the flame.

With such construction, since the wavelength of the light of the light emitting device is substantially fixed, the detection of any flame is made possible irrespective of the kind of fuel such as oil and gas. In this case, the output of a photocell or the like, is applied to the input of an amplifier. The internal impedance of the photocell is small as compared with that of the flame, so that the input impedance of the amplifier need not be very high. Accordingly, the construction of the amplifier becomes easy. Although the input of the amplifier is connected with the photocell by a lead, it is electrically insulated from an electrode in the flame, and hence, the amplifier is hardly susceptible to the influence of heat conducted from the electrode through the lead. Therefore, even when the amplifier is composed of a pure semiconductor circuit, there occur little fluctuations in various characteristics thereof. Such excellent features are attained.

Even the above apparatus, however, has disadvantages as stated below.

Since a stray capacity exists between ground and a wire for connecting the electrode inserted into the flame and the light'emitting device, a closed circuit in cluding a power source, the light emitting device and the stray capacity is formed. As a result, the light emitting device can light up in spite of the absence of the flame.

In case where a discharge tube is employed as the light emitting device, it can light up in the absence of the flame as in the above, due to the stray capacity between the electrodes and envelope of the discharge tube and the stray capacity between the envelope and ground.

Moreover, if the electrical insulation between the electrode to be disposed in the flame and the burner at the time when the flame is not present is inferior, then a malfunction can also occur. The moment a power switch is turned on, the light emitting device sometimes radiates light in spite of the absence of the flame.

SUMMARY OF THE INVENTION It is accordingly the principal object of the present invention to provide novel flame detecting apparatus free from the disadvantages of the prior art as describedabove.

In order to accomplish the object of the present invention, the flame detecting apparatus is characterized, in apparatus in which an electrode inserted into a flame, a light emitting device and a power source are connected in series, in that a capacitor is connected in parallel with the light emitting device.

Owing to the connection of the capacitor, a malfunction attributable to the stray capacitance between the earth and a take-out wire from the electrode (flame rod) can be prevented, and simultaneously therewith, a malfunction attributable to an inferior insulation between the electrode and a burner can be prevented.

Further, the flame detecting apparatus of the present invention is characterized in that a flame rod, a power source, a light emitting device and a burner are connected in series so that one end of the light emitting device and the burner may be grounded in common, and that a capacitor is connected in parallel with the light emitting device.

The other objects, features and advantages will be apparent from the following detailed description when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit diagram showing an embodiment of the present invention;

FIG. 2 is a circuit diagram for explaining the operation of the present invention;

FIG. 3 is a diagram showing the characteristic of the capacity of a capacitor in FIG. 2;

FIG. 4 is a circuit diagram for explaining the operation of the present invention;

FIG. 5 is a circuit diagramshowing another embodiment of the present invention;

FIG. 6 is a circuit diagram for explaining the operation of the embodiment in FIG. 5; and

FIGS. 7 and 8 are circuit diagrams each showing still another embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1 which shows an embodiment of the present invention, numeral 1 designates a burner. Shown at 2 is the flame of the burner, in which are disposed a flame rod 3 for detecting the presence or absence of the flame and a rod 10 for ignition. The flame rod 3 is connected to one terminal X of an AC power 1 source 6 through a series circuit consisting of a protective resistance 4 and a light emitting device 5. The resistance 4 prevents an overcurrent from flowing through the light emitting device 5 when the flame rod 3 and the burner 1 are short-circuited. The other terminal Y of the power source 6 and the burner 1 are both grounded. As the light emitting device 5, there can be used a discharge tube, a light emitting diode, an electroluminescent device etc. which give forth light of fixed wavelengths. In proximity to the light emitting device 5, a photoelectric conversion device 7 such as a photocell is arranged. An electric output obtained by nals of the power source 6. The magnet valve 14 serves to control the feed of gas to besupplied to the burner 1.

In order to raise the photoelectric conversion efficiency, it is desirable to integrally provide the light emitting device 5 and the photoelectric conversion device 7 within one envelope (as depicted by chain lines in the figure).

A timer 13 is connected in parallel with the power source 6. The on-off control of contacts a and b is made by the output of the timer 13. The contact h is connected in parallel with the contact c, while the contact a is interposed between the primary winding of an ignition transformer 11 and ground. A voltage with the voltage of the power source 6 boosted is applied to the rod 10 for ignition from the secondary side of the ignition transformer 11. I

Description will now be made of the operation of the apparatus constructed as described above.

When the power source 6 is turned on for ignition, the timer l3 isactuated to close the contacts b and a. The gas magnet valve 14 is consequently operated, to supply fuel to the bumerl. Simultaneously therewith, the high voltage is applied through the ignition transformer 11 to the igniting rod 10. Accordingly, spark discharge is generated between the rod 10 and the burner 1, to instantaneously ignite the gas supplied through the burner 1. In case of misfiring due to any cause, the contacts b and a are opened by the timer 13 after a predetermined time, so as to interrupt the power supply to the gas magnet valve 14 and the ignition transformer 11. Accordingly, the gas solenoid valve 14 is closed, and the spark discharge between the firing rod 10 and the burner l is stopped.

On the other hand, in case of the instantaneous ignition without misfiring, the part between the rod 3 and the burner l is electrically conducted by the flame 2. Therefore, a closed circuit is formed from the terminal X of the power source 6 via the light emitting device 5, protective resistance 4, rod 3, flame 2, burner l and ground to the terminal Y of the power source, to cause a current to flow therethrough. In consequence, the light emitting device 5 gives forth light, and the resistance of the photocell 7 decreases rapidly. For this reason, a voltage drop across a resistor 20 connected between the base and emitter of the transistor 8 becomes large, to bring into the on state the transistor 8 normally in the off state and to further bring the transistor 12 into the on state. As a result, the relay 9 is energized. The relay contact c is consequently closed, so that even when the timer 13 operates at the predetermined time after turning on the power source 6 and the contact b is thus opened, the current conduction to the gas solenoid valve 14 is held to sustain the combustion.

- In case where the flame is extinguished due to any cause during the combustion, the light emanation of the light emitting device 5 is stopped, and the photocell 7 and the transistors 8 and 12 are reset to the original states. Therefore, the contact a of the relay 9 is opened, the gas magnet valve 14 is closed, and the supply of the gas is stopped.

In the above apparatus, it is possible to insert two,

rods into the'flame and to thereby take out the presence or absence of the flame in the form of a difference in the electric conductivity between both the electrodes. It is also possible to control the opening and closure of the gas magnet valve 14 and simultaneously operate an alarm device as well as a display device by means of the relay 9. I

A feature of the present invention resides in that the power source and the light emitting device are connected in series between the flame rod and the burner or between the two flame rods, while a capacitor 15 is connected across the terminals of the light emitting device 5.

An effect owing to the connection of the capacitor 15 will be described in detail In the absence of the flame 2 in a circuit in FIG. 2, the burner l and the flame rod 3 are electrically insulated as previously stated. Therefore, the light emitting device 5 ought not to radiate.

In actuality, however, when the lead between the flame rod 3 and the light emitting device 5 is long, the stray capacity C between the lead wire and the ground wire increases. Without the capacitor 15, the voltage of the power source 6 will be divided by the stray capacity C,, the protective resistance 4, and the capacity between both the electrodes of the light emitting device 5. The light emitting device 5 gives forth light due to the voltage component applied thereto. By way of example, it is assumed that C, is 10 pF, that C, is 2 pF, and that the voltage E of the AC power source 6 is 100 V. Let E, be a voltage across C,, E, be a voltage across C,,, R be the resistance of the protecting resistor 4, and I be a current flowing through a closed circuit of the terminal X of the power source 6 stray capacity C, protective resistance 4 stray capacity C the terminal Y of the power source 6. Then, the following equation holds:

Since the value of the protective resistance 4 is usually several hundred KQ, it is negligible. Accordingly,

Thus, the voltage of approximately 85 V is applied to the light emitting device 5 even in the absence of the flame. When, for example, the discharge tube is employed as the light emitting device 5, it gives forth light since its firing voltage is about 35 V. This leads to an erroneous operation.

Therefore, the capacitor of, for example, 100 pF is connected in parallel with the light emitting element 5 as in the present invention. Then,

The light emitting device accordingly emits no light.

If the flame is present, a resistance of several M0 several ten MO is connected in parallel with the stray capacity C,. A voltage enough to cause the light emitting device 5 to emit light is therefore impressed on the device 5, so that the flame can be detected without hindrance.

As described above, the malfunction due to the presence of the stray capacity C, can be prevented by connecting the capacitor 15 having a capacity of the value by which the voltage applied to the light emitting device 5 after the division by the stray capacity C, and the capacitor 15 is made lower than the operating voltage of the light emitting device 5, the value being approximately 100 pF or more.

On the other hand, in case where the electrical insulation between the flame rod 3 and the burner l is poor, if the capacitor 15 is not comprised, it is feared that the light emitting device 5 will radiate even in the absence of the flame 2.

More specifically, the voltage of the power source 6 is divided by the internal resistance of the light emitting advantage as above stated can be eliminated.

As is well known, a burning flame has a varying electrical conductivity, and also a rectifying property. That is, the electrical conductivity between the flame 2 and the burner l in one direction is better than that in the other direction. Accordingly, when an AC voltage is applied between the flame rod 3 and the burner l, the magnitudes of currents flowing in the positive half cycle and in the negative half cycle differ. The currents flowing through the part between the flame rod 3 and the burner 1 contain DC components. It, therefore, the light emitting device 5 is operated with only the DC components, it will undergo no malfunction even when the insulating property between the flame rod 3 and the burner 1 becomes inferior.

In order to use the capacitor 15 for this purpose, the capacity of the capacitor need be selected at a value by which the AC voltage across terminals of the capacitor is made lower than the radiation starting voltage of the light emitting device 5 at the frequency of the power source.

For example, it is assumed that the voltage of the power source 6 is 200 V, that the power source frequency is 60 Hz, that the resistance 4 is 300 H] and that a discharge tube is used as the light emitting device 5. Then, the relation between the capacity of the capacitor l5 and the applied voltage of the discharge tube 5 is represented by a curve A shown in FIG. 3. The capacity of the capacitor need exceed a point B beyond which the applied voltage of the light emitting device can be made below the light emission-initiating voltage. That is, a capacity higher than approximately 0.01 uF may be employed.

Owing to the connection of the capacitor 15 of such capacity across the terminals of the light emitting device 5, even when the insulation between the flame rod 3 and the burner 1 is degraded to increase the AC current therebetween, the AC current flows through the by-pass condenser 15, and hence, no influence is exerted on the operation of the light emitting device 5. The connection of the capacitor 15 of the specified capacity can also prevent the malfunction attributable to the foregoing stray capacity C,. With an excessively high capacity of the capacitor, however, the time constant of charging and dischargebecomes excessively large, so that a considerable time is required for the operation of the light emitting device in spite of the presence of the flame. It is accordingly desirable that the value of the capacitor is of the order of 0.001 0.1 p.F. When the flame exists, the DC components are charged in the capacitor 15 by the rectifying effect of the flame. When the terminal voltage of the capacitor reaches the light emission-starting voltage, charges stored in the capacitor are discharged through the light emitting device 5, and cause the light emitting device to radiate.

In the case where the capacitor 15 is not included,

when the flame rod 3 and the burner l are shortcircuited, a malfunction occurs in which the light emitting device gives forth light even in the absence of the flame. In contrast, in case where the capacitor 15 is provided as in the present invention, even when the part between the flame rod 3 and the burner l is shortcircuited, the light emitting device 5 does not radiate since only the AC current flows through the shortcircuited part.

When the capacity of the capacitor connected in parallel with the light emitting device 5 is large, the time constant of charging and discharge is large, and the light emitting device 5 emits light intermittently. As a result, the output of the photocell 7 becomes an intermittent signal, to bring the relay 9 into an intermittent operation. In order to prevent the inconvenience, the apparatus in FIG. 1 has a capacitor 16 connected in the input circuit of the transistor 8, so that the intermittent signal to be applied to the transistor 8 is held till a signal of the next period. Thus, continuous current flows to the relay 9 during the presence of the flame.

7 As stated above, the AC power source 6 and the light emitting device 5 are connected in series between the flame rod 3 and the burner 1, while the capacitor 15 is connected in parallel with the light emitting device 5, whereby the foregoing various malfunctions can be prevented. However, a problem still remains unsolved.

As illustrated in FIG. 4, current flows from the power source 6 to ground through the stray capacity C, be-

tween a lead wire (orelectrode) of the light emitting device 5 and a glass envelope thereof and the stray capacity C between the glass envelope and ground. A slight discharge can therefore take place between the lead wire (electrode) and the glass envelope. It is feared that, when the photocell 7 senses the light of the discharge, the relay 9 willbe operated in-spite of the absence of the flame.

' FIG. 5 shows another embodiment of the present invention which has also solved the above-mentioned disadvantage. In the figure, the same reference numerals are assigned to the same constituting parts as in FIG. 1. Numeral 17 indicates a transformer, the primary winding of which is connected across terminals of the AC power source 6 and the secondary winding of which is connected between one end of the protective resis tance 4 and one end of the light emitting device 5. Shown at 18 is a control circuit, which corresponds to the circuit in FIG. 1 as includes the transistors 8 and 12 and the relay 9. A point of difference of the circuit in FIG. 5 from that in FIG. 1 is that,- since the supply voltage is applied through the transformer 17 across the part between the flame rod 3 and the burner 1, one terminal of the light emitting device 5 is earthed instead of floating from ground.

With such construction, although the stray capacities C and C exist between the lead wire (or electrode) of the light emitting device 5 and ground, the capacitor 15 is connected in the same place. Usually the capacity of the capacitor 15 is extremely large as compared with the stray capacities C, and C so that the latter capacities are negligible. (Refer to FIG. 6.) s

In other words, current flowing from the power source 6 via the transformer 17 into ground passes through the capacitor 15 at its greater part, and scarcely passes through the stray capacities C and C The fear that the discharge will arise between the electrode of the light emitting device 5 and the glass bulb on the basis of the stray capacities C, and C is accordingly eliminated perfectly.

When there is no flame, the secondary voltage of the transformer 17 is divided by the stray capacity C, and the capacity of the capacitor 15, and the divided voltage component of the capacitor 15 is applied across both the terminals of the light emitting device 5. If the capacity of the capacitor 15 is selected to be sufficiently larger than the capacity C,, the applied voltage can be suppressed to a value lower than the light emission-initiating voltage of the light emitting device 5. Accordingly, the malfunction in which the light emitting device 5 gives forth light in spite of the absence of the flame can be prevented as in the apparatus in FIG. 1

which it functions as a by-pass condenser with respect to the frequency of the AC power source, the light emitting device 5 can be operated by DC components by virtue of the rectifying effect the flame. In this case, even when the flame rod 3 and the burner l'have the insulation deteriorated or are short-circuited, no malfunction occurs. Furhter, in this case, the light emitting device 5 gives forth light intermittently due to the charging and discharge of the capacitor 15. In order to convert the inten'nittent light emission into a continu'-' ous electric signal, the holding capacitor 16 may be connected to the input circuit of the transistor 8 as in the apparatus in FIG. 1.

As in the foregoing, the discharge tube, light emitting diode, EL element or the like can also be used as the light emitting device 5 in the embodiment in FIG. 5.

FIG. 7 shows still another embodiment of the present invention. According to the embodiment, two flame rods 3a and 3b are disposed in a place in which the flame 2 is to exist, they are respectively connected to the terminals of the secondary winding of the power transformer 17, and the light emitting device 5 is connected between an intermediate tap of the secondary winding and ground. 7

The flame has the rectifying action as previously stated. With the construction in FIG. 7, therefore, the secondary voltage of the transformer 17 is subjected to full-wave rectification, and the DC components are charged up in the capacitor 15. When the terminal voltage of the capacitor 15 reaches the light emissionstarting voltage of the light emitting device 5, charges stored in the capacitor are discharged through the device 5. v

Since, in accordance with such construction, the supply voltage is effectively utilized, it is possible to shorten the time until the radiation starting voltage is reached after initiation of the charging of the capacitor 15, namely, the period in which the light emitting device does not yet give forth light after the initiation of thecharging. I

FIG. 8 shows a furtherembodiment of the present invention, in which a resistor 19 is connected in series with the light emitting device 5, and the capacitor 15 is connected across both ends of the series circuit.

According to such construction, charges stored in the capacitor 15 are discharged through the resistor 19 and the light emitting device 5, so that the discharge time and accordingly the light emitting time can be made long. More specifically, in the absence of the resistor 19, the discharge proceeds in a moment, and the light emitting period of the light emitting device is short.

When the capacitor 15 is selected at a capacity at Consequently, the photocell 7 sometimes fails to detect the light emission. Such disadvantage can be eliminated by connecting the resistor 19 for adjustment of the discharge time constant as stated above.

If, in each of the foregoing embodiments, the light emitting device and the photocell 7 are made monolithic into a composite element, the photoelectric conversion efficiency is raised.

The present invention having thus far been described is applicable to a wide range of uses as in a kerosene burner equipment, heavy oil burner equipment, instantaneous water boiling unit, gas bath, kerosene stove, gas furnace, etc., and can advantageously provide a flame detector of reliable operation and low cost. In case where there are a number of stoking equipments, the remote centralized observation of burning is'also possible.

We claim:

1. Flame detecting apparatus comprising a burner, at least one flame rod arranged in a place in which a flame is to exist, an AC power source and a light emitting device which are connected in series between said burner and said flame rod, and a capacitor connected across both terminals of said light emitting device and in parallel therewith, whereby the presence or absence of the flame is detected in the form of the presence or absence of a radiation output of said light emitting device.

2. The flame detecting apparatus according to claim 1, further comprising photoelectric converter means to convert the radiation output .of said light emitting device into an electric signal, and means to control the supply of fuel to said burner in response to an output signal of said converter means.

3. The flame detecting apparatus according to claim 1 wherein said capacitor has a capacity selected to be sufficiently larger than a stray capacity between the earth and a lead wire connecting said flame rod and said light emitting device.

4. The flame detecting apparatus according to claim 1, wherein said capacitor has a capacity selected to present a sufficiently low impedance at a frequency of said AC power source, in order to charge therein only DC components generated by the rectifying action of the flame.

5. The flame detecting apparatus according to claim 2, wherein a second capacitor for changing an intermittent electric signal, being the output of said photoelectric converter means, into a continuous signal is connected between said photoelectric converter means and the fuel supply control means.

6. The flame detecting apparatus according to claim 1, wherein a resistor for making the discharge time constant of said capacitor long is connected in a closed circuit including said light emitting device and said capacitor.

7. Flame detecting apparatus comprising a burner, a flame rod arranged in a place in which a flame is to exist, a light emitting device to radiate in response to the presence of the flame, one of electrodes of said device being connected to said burner, and power source means including a source for generating an AC voltage and a transformer whose primary winding is connected across said source and whose secondary winding is connected at one end to said flame rod and at the other end to the other electrode of said light emitting device.

8. The flame detecting apparatus according to claim 7, further comprising photoelectric converter means to convert the radiation output of said light emitting device into an electric signal, and means to control the supply of fuel to said burner in response to an output signal of said photoelectric converter means.

9. The flame detecting apparatus according to claim 7, wherein said light emitting device is a discharge tube, and said-capacitor has a capacity selected to be suffciently larger than a stray capacity between an electrode of said discharge tube and a glass envelope thereof and a stray capacity between said glass envelope and the earth.

It). The flame detecting apparatus according to claim 7, wherein said capacitor has a capacity selected to present a sufficiently low impedance at a frequency of said AC power source, in order to by-pass AC currents and to charge therein only DC components generated by the rectifying action of the flame.

1 l. The flame detecting apparatus according to claim 8, wherein a second capacitor for changing an intermittent electric signal, being the output of said photoelectric converter means, into a continuous electric signal is connected between said photoelectric converter means and the fuel supply control means.

12. The flame detecting apparatus according to claim 7, wherein a resistor for making the discharge time constant of said capacitor long is connected in a closed circuit including said capacitor and said light emitting device.

13. Flame detecting apparatus comprising a burner, two flame rods arranged in a place in which a flame is to exist, AC power source means to supply an AC voltage across said two flame rods and including a source for generating an AC voltage and a transformer whose primary winding is connected across said source and whose secondary winding is connected at both ends to said two flame rods, a light emitting device connected between an intermediate tap of said transformer and said burner, and a capacitor connected across both terminals of said light emitting device and in parallel therewith, whereby the presence or absence of the flame is detected in the form of the presence or absence of a radiation output of said light emitting device.

14. The flame detecting apparatus according to claim 1, wherein said light emitting device is a discharge tube.

15. Flame detecting apparatus comprising a plurality of flame rods arranged in a place in which a flame is to exist, an AC power source and a light emitting device which are connected in, series among said flame rods,

and a capacitor connected across both terminals of said light emitting device and in parallel therewith, whereby the presence or absence of the flame is detected in the form of the presence or absence of a radiation output of said light emitting device.

16. The flame detecting apparatus according to claim 1, wherein said capacitor has a capacity ranging from 0.001 to 0.1 [.LF. 

1. Flame detecting apparatus comprising a burner, at least one flame rod arranged in a place in which a flame is to exist, an AC power source and a light emitting device which are connected in series between said burner and said flame rod, and a capacitor connected across both terminals of said light emitting device and in parallel therewith, whereby the presence or absence of the flame is detected in the form of the presence or absence of a radiation output of said light emitting device.
 2. The flame detecting apparatus according to claim 1, further comprising photoelectric converter means to convert the radiation output of said light emitting device into an electric signal, and means to control the supply of fuel to said burner in response to an output signal of said converter means.
 3. The flame detecting apparatus according to claim 1, wherein said capacitor has a capacity selected to be sufficiently larger than a stray capacity between the earth and a lead wire connecting said flame rod and said light emitting device.
 4. The flame detecting apparatus according to claim 1, wherein said capacitor has a capacity selected to present a sufficiently low impedance at a frequency of said AC power source, in order to charge therein only DC components generated by the rectifying action of the flame.
 5. The flame detecting apparatus according to claim 2, wherein a second capacitor for changing an intermittent electric signal, being the output of said photoelectric converter means, into a continuous signal is connected between said photoelectric converter means and the fuel supply control means.
 6. The flame detecting apparatus according to claim 1, wherein a resistor for making the discharge time constant of said capacitor long is connected in a closed circuit including said light emitting device and said capacitor.
 7. Flame detecting apparatus comprising a burner, a flame rod arranged in a place in which a flame is to exist, a light emitting device to radiate in response to the presence of the flame, one of electrodes of said device being connected to said burner, and power source means including a source for generating an AC voltage and a transformer whose primary winding is connected across said source and whose secondary winding is connected at one end to said flame rod and at the other end to the oTher electrode of said light emitting device.
 8. The flame detecting apparatus according to claim 7, further comprising photoelectric converter means to convert the radiation output of said light emitting device into an electric signal, and means to control the supply of fuel to said burner in response to an output signal of said photoelectric converter means.
 9. The flame detecting apparatus according to claim 7, wherein said light emitting device is a discharge tube, and said capacitor has a capacity selected to be sufficiently larger than a stray capacity between an electrode of said discharge tube and a glass envelope thereof and a stray capacity between said glass envelope and the earth.
 10. The flame detecting apparatus according to claim 7, wherein said capacitor has a capacity selected to present a sufficiently low impedance at a frequency of said AC power source, in order to by-pass AC currents and to charge therein only DC components generated by the rectifying action of the flame.
 11. The flame detecting apparatus according to claim 8, wherein a second capacitor for changing an intermittent electric signal, being the output of said photoelectric converter means, into a continuous electric signal is connected between said photoelectric converter means and the fuel supply control means.
 12. The flame detecting apparatus according to claim 7, wherein a resistor for making the discharge time constant of said capacitor long is connected in a closed circuit including said capacitor and said light emitting device.
 13. Flame detecting apparatus comprising a burner, two flame rods arranged in a place in which a flame is to exist, AC power source means to supply an AC voltage across said two flame rods and including a source for generating an AC voltage and a transformer whose primary winding is connected across said source and whose secondary winding is connected at both ends to said two flame rods, a light emitting device connected between an intermediate tap of said transformer and said burner, and a capacitor connected across both terminals of said light emitting device and in parallel therewith, whereby the presence or absence of the flame is detected in the form of the presence or absence of a radiation output of said light emitting device.
 14. The flame detecting apparatus according to claim 1, wherein said light emitting device is a discharge tube.
 15. Flame detecting apparatus comprising a plurality of flame rods arranged in a place in which a flame is to exist, an AC power source and a light emitting device which are connected in series among said flame rods, and a capacitor connected across both terminals of said light emitting device and in parallel therewith, whereby the presence or absence of the flame is detected in the form of the presence or absence of a radiation output of said light emitting device.
 16. The flame detecting apparatus according to claim 1, wherein said capacitor has a capacity ranging from 0.001 to 0.1 Mu F. 